I originally thought that this is a duplicate of ticket #2559 but since this is a regression since 6adf97f, I don't think so.

The scale filter has to be inserted twice because yadif does not support the input pixel format and you generally cannot resize before deinterlacing. For the specific case (input pal8, yadif, resize, output yuv420p), the problem can be fixed imo because the trivial choice works, for other encoders, it would be more difficult to see the right pix_fmt.